1. Field of the Invention
The present invention relates to an optical disc having formed thereon lands and grooves, which form together a data recording track, and an optical disc drive for the optical disc.
2. Description of Related Art
Well-known data recording media include optical discs such as a magneto-optical type, phase-change type and the like. They include a read-only ROM disc, a recordable disc, a recordable/reproducible RAM disc, a so-called partial ROM disc having a ROM area and a RAM area, etc.
In addition, some of the optical discs called land/groove recording type optical discs, have been proposed which are designed to record data at both lands and grooves for a larger capacity of data storage.
In these conventional optical discs, a recording track is wobbled to provide a servo control information and address information (ADIP) for data recording and reproduction. In the conventional optical discs, however, a recorded data, when reproduced, will be deteriorated due to an amount of light varied, and a light polarizing direction disturbed, by the wobbling, and thus will show a poor signal-to-noise ratio. The influence of this wobbling on a data recording into an optical disc with a high density using a large NA, for example, is not negligible.
For a higher density of data recording into an optical disc, a clock mark is necessary which is highly reliable and can reproduce a data-independent clock. FIG. 1 shows a conventional clock mark by way of example. It is inserted in a wobble signal. The clock mark is formed as a pattern having a greater length than the diameter of a focused laser spot and wobbled towards the outer and inner circumferences, alternately, radially of an optical disc, so that a varied amount of light will result in an S-shaped signal waveform. Thus, a clock reproduced from such a clock mark is not influenced by MTF of a reproducing optical system and has a good signal-to-noise ratio.
For detecting a clock from such a clock mark, a radial push-pull signal is used. Thus, the clock generated from the clock mark will be affected by a tracking offset and radial tilt of an optical disc. Therefore, no stable clock can be reproduced from the clock mark if a tracking error or the like takes place.
Also, it is desired to improve the accuracy of clocking by increasing the number of clocks. However, a clock mark having a correspondingly increased length will greatly cause data to be recorded with a greater redundancy, thus no higher density of data recording can be attained.
Further, for a higher density of data recording into an optical disc, it is desired to reduce the thickness of the substrate of an optical disc while increasing the NA for irradiation to the optical disc. However, such a reduced thickness of the optical disc substrate will greatly affect the disc substrate itself. Namely, the disc substrate will be deflected or distorted due to a variation of environmental conditions. In addition, the manufacturing cost for such an optical disc with a reduced substrate thickness will be greater because it will include costs for prevention of such environmental influences. To avoid such deformation of the optical disc, an optical disc drive may incorporate a disc tilt detecting mechanism to correct a relative angle between a laser beam and main side of an optical disc. However, the disc tilt detecting mechanism will add to the manufacturing costs for such an optical disc drive.